A known probe 1 for measuring in situ the deformation moduli of the soil layers, commonly referred to as a Flat Dilatometer, is known from U.S. Pat. No. 4,043,186. This probe 1, shown in FIG. 1, comprises a dilatometer blade 2 having inside a pressure chamber in communication with the outside via an opening sealed by a thin circular expandable steel membrane 3 mounted flush on one face of the blade (or equipped with two membranes, one on each face of the blade). The probe is forced vertically into the ground by push rods 4 connected above the blade 2. The dilatometer blade 2 also comprises electrical contacts acting in response to the movement of said metal membrane. The blade 2 is connected to the surface by an internally wired gas conduit 5,6 comprising a gas conduit 5, for example a plastic tube, containing a single wire 6, which runs inside the push rods 4. The gas conduit 5 is connected, at the soil surface, to an external circuit comprising an unit for introduction and removal of compressed gas to and from the chamber dilatometer blade 2, while the single wire 6 allows the electrical communication from the blade 2 to the external circuit. At selected depths the blade 2 is stopped and the membrane 3 is inflated by feeding gas pressure to the blade 2. At each depth the user determines the pressure Po which is the pressure causing the initial lift-off of the membrane, and subsequently the pressure P1 which is the pressure producing a central displacement of the membrane by a predetermined amount, generally 1.1 mm. The instants at which Po and P1 have to be taken are signaled by the opening or the closure of a circuit, determined by the position of the membrane, which works like an ordinary electrical switch. The two poles of the switch are connected to the surface by the live wire inside the gas conduit and the electrical earth i.e. the pushrods. On the surface, a battery and a buzzer (emitting a sound when the circuit is closed) complete the circuit. The two pressures Po and P1 are then interpreted to derive soil parameters.
The known Flat Dilatometer probe has been increasingly used in recent decades. However the Flat Dilatometer determines only static soil parameters, while today there is also a growing interest in seismic parameters, in particular in the shear wave velocity Vs and in the initial shear modulus Go (derivable from Vs).
Various methods are currently available for determining the Vs profile. Among these, frequently used methods are the Cross-Hole and the Down-Hole method, carried out in specially made dedicated boreholes. However these methods require additional field work, hence the global cost and time are substantially higher compared with a situation wherein static and seismic parameters are obtained from just one sounding with the same probe.
This is the reason why research efforts have been carried out in the past (Hepton “Shear Wave Velocity Measurements during Penetration Testing”, Proc. Penetration Testing in the UK, ICE1988, pages 275-278, and G. Martin and P. Mayne “Seismic Flat Dilatometer Tests in Piedmont Residual Soils” Geotechnical Site Characterization 1998 Balkerna, Rotterdam) to combine the Flat Dilatometer with a seismic module for obtaining at the same time the static and seismic parameters.
FIG. 2 shows a known experimental composite probe equipped with a seismic module 7. The seismic module 7 comprises a tubular element 4a, for example a steel tube, connected above the dilatometer blade 2. Inside this tubular element 4a, two receivers 8, 9, for example geophones or accelerometers are placed, spaced typically 0.5 m or 1 m apart.
Each receiver 8, 9 is connected to the surface by a respective wire 10, 11. Each of these wires 10, 11 runs inside the push rods 4 in parallel to the internally wired gas conduit 5,6. Both the internally wired gas conduit 5,6 and the seismogram wires 10, 11 reach the surface and are connected to an external circuitry.
The composite probe 12, formed by the seismic module 7 and the dilatometer blade 2, is forced vertically into the soil by push rods 4 connected above the tubular element 4a of the seismic module 7. At the desired depths the probe 12 is stopped and the operator can either perform measurements of static soil parameters—as described above—or can perform a Vs measurement.
At the depths where the Vs measurements have to be carried out, the probe 12 is stopped and a seismic wave W is generated at ground surface by a source, often a pendulum hammer which hits horizontally a parallelepiped anvil. The seismic wave W propagates downwards and reaches first the upper receiver 8, then the lower receiver 9. The delay between the first and second seismogram is generally determined using the well known cross-correlation algorithm.
Once the delay is known, the shear wave velocity Vs is obtained as the ratio between the easily calculated difference in distance between the source at the surface and the two receivers and said delay. Vs may then be converted into Go, the initial soil shear modulus, by using the theory of elasticity formula Go=ρVs2.
When the combined probe 12 is used for static measurement, the Flat Dilatometer works as previously described and is connected to the surface by its internally wired gas conduit 5,6.
The seismic module 7 transmits in analog form to the surface the electrical seismograms generated by the two receivers 8,9 via wires 11, 10. The seismograms are analyzed at the surface by an oscilloscope to determine the delay.
Such combined probes 12 have produced interesting research results. In particular the obtained results have been the starting point of studies aimed at combining the low strain shear modulus (from the Seismic test) and the operative modulus (from the Flat Dilatometer) for defining the decay curves of modulus versus strain, necessary to perform non linear analysis of the soil deformation under load.
However, the known combined probe 12 has a number of serious practical drawbacks for industrial use in the field of the soil investigations.
One inconvenience is that the wires 10, 11 transmitting the seismograms in an analog form act like antennas and pick up electrical disturbance due to traffic, motors, electrical lines, telecommunication lines and the like. This inconvenience is especially grave at large depths, where the seismograms are weak—due to the distance from the energizing source—and the electrical noise can obscure the signal running on the wires 10, 11.
An even more serious practical inconvenience is the presence of multiple cabling. The presence of more than one cable, in particular either the internally wired gas conduit 5,6, and the wires 10, 11, enormously complicates field testing, as it is well known to experienced operators. The cables 5, 6, 10, 11 have to be threaded inside the push rods 4 and frequently manipulated. The risk of garbling and mixing up two (or more) cables is high, resulting in considerably slower operations and considerably higher overall costs.
In the case of deep investigations in the ground, several cables 5,6, 10, 11 must be must be connected in sequence to obtain the necessary length. The junctions become numerous and the multiple joints, pneumatic and electrical are highly complicated and voluminous.
In the special case of offshore investigations, which are very important to industry, the use of a multiplicity of wire or cables is virtually impossible. In fact in many offshore configurations, the dilatometer blade 2 connected to the internally wired gas conduit 5,6 is suspended by a rope that is even difficult to handle with just a single cable or conduit because the suspended dilatometer blade 2 can rotate, thereby twisting the internally wired gas conduit 5,6, while the rope and the internally wired gas conduit 5,6 must freely slide longitudinally independently of each other, in order to allow the insertion of the dilatometer blade 2 in the soil at the bottom of the sea. It is a well known fact that operators are in many cases obsessed by the problems posed by the presence of wires.
The overcoming of these inconveniences, in particular the multiple wire problem and advantageously the antenna effect, are solved by the embodiments of the present disclosure.